Rechargeable batteries are increasingly becoming an important source of clean portable power in a wide variety of electrical applications, including use in automobiles, boats and electric vehicles. In particular, the lead-acid battery, while an old technology, continues to constitute a substantial portion of the rechargeable batteries being produced. While not particularly energy efficient because of the inherent weight of lead with respect to other metals, lead-acid batteries still retain the advantage of being very power efficient while being among the cheapest, most reliable and most readily producible rechargeable batteries. Thus, lead-acid batteries are typically used to start engines, propel vehicles such as golf carts, and to act as non-interruptable sources of back-up power when an external supply of electricity is interrupted.
The ability of the lead-acid battery to deliver large amounts of electrical power is well known, particularly when associated with the starting of motor vehicles. Likewise, the need to recharge these batteries and the problems associated therewith are also well known.
Many limitations and faults found in lead-acid batteries and other types of batteries are the result of poor recharging control. For example, overcharging of a battery wastes energy, reduces the product life, and may permanently damage the battery. In addition, overcharging can accelerate grid corrosion, increase the specific gravity of the electrolyte by dissociating water into its component gasses, and generate undue heat that tends to amplify and accelerate other problems.
On the other hand, undercharging the battery limits its capacity and likewise leads to degradation of the battery that is often unrecoverable. Undercharging is known to cause stratification of the electrolyte (particularly in flooded batteries), uneven use of the active materials, and may even lead to permanent sulfation of the active materials.
As defined in a reference text entitled "Lead-Acid Batteries," by Hans Bode, (copyright 1977 by J. Wiley and Sons), the capability of a cell or battery to store the charging current so that it can be redischarged is called the charge acceptance. The charge efficiency is defined as the current input actually charging the battery divided by the total input current. Thus, the current flowing into a battery is either used to charge the battery, i.e., the charge acceptance, or must be dissipated, which primarily occurs in side chemical reactions such as gassing. Thus, an idealized instantaneous charge acceptance of a battery at any point in its recharge is a charging current at which all of the available soluble discharge product, i.e., lead sulfate (PbSO.sub.4) for a lead-acid battery, is being charged without undue gas generation. In other words, it is the maximum amount of charging input current that can flow into the battery while still maintaining an acceptable amount of gas generation. Note that since the amount of gas generation that is acceptable to a particular application varies depending on a number of user-determined factors, there is no actual ideal charge current, only a current that most efficiently charges the battery while maintaining an acceptable or desirable amount of gas generation.
To recharge a battery in accordance with its optimized charging requirements ordinarily requires consideration of a large number of factors and compensation therefor. For example, a number of factors such as the intended use of the battery, its age and history, and in particular the resistance and internal temperature of the battery, influence charging requirements. Other factors that are ordinarily considered include the type of battery (such as maintenance free or user maintained), the size of the battery, the rate and amount of discharge, the stand time of the battery since discharge, composition of the battery and the presence or absence of chemical species or impurities that may affect gassing or hinder the solubility of lead sulfate. Finally, safety must always be considered since fire, fuming, explosion, or thermal runaway may occur as a result of uncontrolled excessive overcharging.
In addition, consideration must be given to whether the charging apparatus is intended to be a stand-alone unit or must be capable of functioning in a vehicle charging apparatus. More particularly, in a vehicle, the charging may be taking place alternatively as the battery may be called on to deliver power under a wide range of circumstances.
As a result of these and other potential charging considerations, the methods of charging lead-acid (and other) batteries have traditionally fallen into two primary categories. The first category consists of a constant current apparatus and method which applies an arbitrarily low current to a discharged battery until it is fully recharged, typically taking ten-to-twenty hours. The current value is purposely set low so that it will not harm the battery at the end of the charging period. Particularly when dealing with flooded batteries, this charging technique is frequently supplemented with an excess of charge which destratifies the electrolyte with gasses produced from the electrolysis of water.
However, in addition to taking a considerable amount of time to recharge, if the level of discharge prior to recharging is not initially known, the time and amount of current necessary to properly recharge the battery will be difficult to establish and the battery must therefore be monitored to prevent undercharge or excess overcharge. Numerous prior techniques have attempted to measure the point at which to terminate this type of time-inefficient charging, but all suffer from the same inherent problem, i.e., having constant current charging does not match the variable battery charge acceptance requirements throughout the recharging process.
The second category of charging methods and apparatuses are those which are based on a constant or fixed voltage output. Chargers having this capability are usually set to a maximum initial current output, which remains at that level until the voltage of the battery rises to that of the charger, whereby the current tapers gradually to a low end value. A full charge is indicated by a low, steady current. These chargers are typically left on until the battery is needed for use or when a maximum time limit has been reached.
Because of the low amount of current at the end of the charging cycle and the low overcharge at typically conservative recharge voltages, with this type of apparatus a deeply discharged flooded battery often will have a stratified electrolyte even when full recharge is obtained, limiting battery performance and life. Increasing the voltage level in order to achieve more gassing or agitation and mixing is dangerous and difficult to accomplish without severely overcharging the battery if applied for too long of a period of time and possibly leads to thermal runaway. Other problems that arise with this type of charger include high battery temperatures and impurities that significantly influence the amount of charging current flowing to the battery at a fixed voltage. Accordingly, numerous techniques have been developed for constant voltage chargers that attempt to determine temperature compensation and the point of termination of charge, but again fail to match the optimal charge acceptance needs at any given moment.
Hybrid combinations of both of these charging methods with various timing mechanisms still suffer from the basic limitations imposed by these types of chargers, that is, they arbitrarily set the parameters for battery charging which restricts the conditions under which they work. In other words, the charging mechanism, not the battery itself, dictates the amount of charge supplied.
As a result of these and other well-known charger control problems, long periods of recharge at low rates of charging, known as trickle charging, have historically been used for lead-acid batteries, and are still common. However, with this recharging method the battery is tied up for long periods of time, thus greatly reducing the amount of time available for utilizing the battery. This is particularly unacceptable in applications where short recharge times are almost essential, such as with battery-based electric vehicles.
The need for a high rate of recharge is therefore paramount to efficient battery use in many applications. However, applying relatively high recharge currents, even for short periods of time, must be very carefully controlled, or will lead to excessive gassing, electrolyte spewing and increased heating that are both dangerous and can cause permanent battery damage and reduced life.
Most recently, microprocessor-based systems have been employed to control recharging techniques in various attempts at adjusting battery chargers to the changing conditions of the battery during charging. To function, however, these systems require extensive data calculations and suppositions for many battery conditions. For example, probing cycles are regularly made by such systems during charging in order to calculate a more ideal current or voltage charging level. However, these probing cycles (to determine improvements to charging conditions) tend to be lengthy in order to obtain useful data and are normally performed at a less-than-optimal charge. Thus, such systems cannot achieve the level of control required on the instantaneous basis that is required during high rates of charging. Moreover, frequent probing cycles can even lead to reduced control and efficiency.
The ability of a charging technique that inherently compensates and adjusts for such factors as the type of battery, temperature, previous discharge rate, stand time, state of charge and other such factors will lead to a very efficient battery charger and improved battery life. In spite of numerous prior attempts to determine an optimized electrical charging output required to match the charge acceptance needs of a recharging battery, no simple, reliable, rapid, and safe way to provide a variable recharging signal as dictated by the charge acceptance of the battery throughout the charging cycle is currently available.
Accordingly, it is a primary object of the present invention to provide a method and apparatus for charging a battery that regularly adjusts its electrical charging output to approach a more optimized level of charge acceptance of the battery.
It is a related object to provide a battery charger that inherently compensates for factors influencing charge acceptance requirements.
Another object of the invention is to provide a simple and efficient method and apparatus for charging a battery that automatically adjusts to determine an optimized recharging current and voltage profile for the battery based on the requirements of that specific battery and the output capacity of the charger.
An additional object is to provide a battery charger that is capable of efficiently and safely charging batteries of various types and sizes while controlling the amount of gassing.
Still another object is to provide a simple and reliable battery charger that can be combined with other charging techniques for handling diverse conditions and controlling the phases of a charging operation.